With a machine tool such as a lathe, when a processed work gripped by a spindle chuck is replaced with a new one by a loader or the like, rotation of a spindle is stopped, and the loader receives the processed work. The loader then passes a new work to the spindle chuck. Thereafter, the spindle motor is rotated to start up the spindle so as to increase the rotation speed of the spindle up to a value required for processing. Thus, replacement of the work on the spindle chuck requires an amount of time for decelerating and stopping the spindle, an amount of time for delivering the work, and an amount of time for starting up the spindle so as to appropriately rotate the spindle. This work replacement time is a factor prolonging processing cycle time. In particular, for processing in which only a short time is required to process the work by means of a tool contacting with the work, the work replacement time accounts for most of the cycle time.
In a proposed lathe allowing a reduction in work replacement time, two spindles are provided on a spindle head so that the each of the spindles can be switched between a processing position and a standby position; while a work on the spindle located at the processing position is being processed, a work on the spindle located at the standby position is replaced with a new one (see, for example, the Unexamined Japanese Patent Application Publication (Tokkai-Sho) No. 58-196904). One spindle motor is used to drive the spindles and power is transmitted to the spindle located at the processing position.
Another lathe has been proposed in which a spindle head with a spindle and a spindle motor supported thereon are installed on each of the opposite sides of one tool rest so that while a work on a first spindle is being processed, a work on a second spindle is replaced with a new one, and the second spindle is started up so as to rotate appropriately (see, for example, the Unexamined Japanese Patent Application Publication (Tokkai-Hei) No. 2001-341001).
In the proposed example in which the two spindles are provided and rotated by the common spindle motor, the processed work and the new work can be passed to and from the spindle chucks during the processing time. However, the operation of starting up the spindle so as to appropriately rotate the spindle needs to be performed after the spindle located at the standby position reaches the processing position. This prevents a reduction in the time required to start up the spindle so as to appropriately rotate the spindle, which time is included in the work replacement time.
The above-described lathe including the two spindles and the two spindle motors is excellent in reducing the replacement time. However, the lathe needs to include the two spindles, the two spindle motors, and the two spindle heads, thus requiring a large-scale facility and high facility costs.